Illuminating devices of the type described are known, for example, from German Patent Application DE 199 31 954 A1. This illuminating device is used as a light source and wavelength selection device for a DUV (=deep ultra-violet) microscope. Illuminating devices for DUV microscopes must provide illuminating light of a narrow illumination wavelength range for which the microscope optics are corrected. The illumination wavelength range is characterized by the spectral position of the intensity maximum and the half-value width thereof. Known systems for selecting the desired illumination wavelength range from a spectrum of a suitable light source, such as a mercury vapor lamp, include both narrow-band transmission filter systems and reflecting filter systems. These filter systems are arranged in the illumination beam path and feed light of the selected to the illumination wavelength range to the microscope as useful light.
Narrow-band transmission filter systems in the DUV provide peaks having a very narrow half-value width; however, the maximum transmission and, thus, the maximum value of the peaks, is only about 20% of the input optical power present upstream of narrow-band transmission filter system. Therefore, narrow-band transmission filter systems do not constitute efficient wavelength selection devices for DUV light.
The known reflecting filter systems are composed of several reflecting filters on which the light of the light source is incident at a certain angle of incidence and at which it is reflected. In these reflecting filter systems, the reflected and thus useful component of the light of the selected illumination wavelength range is considerably more than 90% of the input optical power.
In the reflecting filter system known from German Patent Application DE 199 31 954 A1, provision is made for an arrangement of reflecting filters in which the angles of reflection at the individual reflecting filters are smaller than 30°. This makes it possible to produce a half-value width of the selected illumination wavelength range of smaller than 20 nm. Using such a reflecting filter system with small angles of incidence, the optical power exiting the reflecting filter system can be about 98% of the input optical power, depending on the type of reflecting filters used.
FIG. 1 from German Patent Application DE 199 31 954 A1 shows such an illuminating device together with a DUV microscope. In this context, the reflecting filter system is arranged between the light source and the DUV microscope, which by itself requires an enormous amount of space. If the DUV microscope is used as a semiconductor inspection microscope, it must be installed and operated in a clean room. Since the expenditure for operating a clean room increases considerably with increasing volume, the equipment installed there must be arranged in the most space-saving way possible, ideally requiring only a small footprint.
It is often necessary to use several illuminating light sources in an optical system and particularly in a microscope system, the illuminating light sources in each case having an illumination wavelength range extending from the infrared via the visual to the DUV wavelength ranges. To this end, usually, up to three different light sources along with the lamp housings are integrated opto-mechanically, which can only be accomplished using a number of deflection mirrors and/or points of beam combination in the illumination optics and disadvantageously involves an enlargement or increase in the number of optical beam paths.